It looks like XCTest is missing some basic support for testing if a function throws or not. Here are two snippets that I'm using until that support is added.

func XCTAssertDoesNotThrow(@autoclosure fn: () throws -> (), message: String = "", file: String = __FILE__, line: UInt = __LINE__) {
    do {
        try fn()
    }
    catch {
        XCTFail(message, file: file, line: line)
    }
}

func XCTAssertDoesThrow(@autoclosure fn: () throws -> (), message: String = "", file: String = __FILE__, line: UInt = __LINE__) {
    do {
        try fn()
        XCTFail(message, file: file, line: line)
    }
    catch {
    }
}

I'm not sure how to extend this to validating for a specific error though…

Using it in a test is pretty easy:

func f() throws {
}

func someTest() {
    XCTAssertDoesNotThrow(try f())
}